This invention relates to a method and apparatus for sensitive high speed spectroscopic gas detection over a wide range of gas concentrations from less than one (1) part per million (ppm) up to 100%.
In general, the transmission or absorption of radiation passing through a radiation absorbing medium, such as a gas, may be graphed as a function of wavelength or frequency as an optical absorption spectrum. The absorption spectrum consists of a set of discrete absorption lines characteristic of the gas of interest. In general, absorption measurements within a narrow frequency interval and comparable or smaller than the line width near an absorption peak can be used as a sensitive measure of the number or concentration of absorbing gas molecules present. At very large concentrations, all of the radiation within a small frequency interval near an absorption peak is heavily absorbed. This is called absorption line saturation. However, when measurements are made away from the peak absorption frequency or over a wider spectrum where the frequency interval of measurement is larger, the absorption measurement includes contributions from frequencies which are not saturated. These off-line absorption measurements are highly dependent on the shape of the absorption lines. Line shape dependent measurements include the integrated absorption over an entire line or multiple absorption lines grouped into bands which are characteristic of the absorbing species.
A typical method of measuring gas concentration relies upon absorption of electromagnetic radiation by the target gas atoms or molecules when electromagnetic radiation is passed through a sample containing a gas which absorbs some of the radiation. However, such known methods are limited with respect to the range of concentrations over which the target gas atoms or molecules can be detected.